Can we beat the summer heat?

By Alexandra Stinemetz, Ph.D., Pioneer field agronomist

Alexandra Stinemetz, Ph.D., Pioneer field agronomist

The rainfall pattern across Ohio during the early part of the 2024 growing season has been irregular. The precipitation kept fields wet in many of the northwestern counties delaying planting until late May and early June. The state has witnessed a variety of weather conditions but the above average temperatures and accelerated growing degree unit (GDU) accumulation has been experienced by all. What impact will high temperatures have on our corn and soybean crops?

Warm weather causes corn to grow faster. Optimal daytime temperatures for corn ranges between 77 degrees F and 91 degrees F. Growth decreases when temperatures exceed 95 degrees F but even temperatures in the mid-90s are not a problem until soil moisture is lacking. Leaf rolling is thought to be the first sign of drought stress in corn. The extent of corn yield loss is determined by stage of growth and the duration of plant wilting. Additionally, signs that a corn field is experiencing severe drought include: corn tissue with a gray color, overall plant height may be shortened and lower leaves may begin to yellow and even turn brown and die.

While an early drought has shown less impact on overall yield, it does not come without some physiological impact. As the corn crop is developing nodal roots between V1 and V5, extreme drought can impact the development of these roots and result in what is known as “floppy” or “rootless” corn. As the plant moves into V6-V8 growth stages, the crop is readily determining the number of rows on the corn ears. A severe drought leading up to or during these growth stages can result in more slender ears with fewer rows around.  

This table shows the estimated corn yield loss when drought stress persists for 4 or more consecutive days.

Also note, nutrients that are more mobile may show deficiency with prolonged drought stress. Mobile nutrients such as nitrogen and potassium are needed in larger quantities and are usually the first to show up as a deficiency on lower leaves. As the crop moves into reproductive stages, many yield critical functions require ample moisture. Activities such as pollen fertilizing the ovules and the process of filling out corn ears both require sufficient moisture. This is why you will notice the greatest yield impact when severe drought has been present for over 4 days from pollination through dough stage.

Soybeans, like corn, will experience much less of a yield impact from an early vegetative drought compared to dry weather during reproductive stages. Some of the physiological responses that you may observe during a drought in soybeans include: decreased leaf size, leaf flip — to reflect sunlight and conserve water — and leaf clamping. Soil temperatures of 90+ degrees F for prolonged periods can result in a decrease of nodulation and nitrogen fixation. However, if the deficit of water is only short-lived nitrogen fixation may resume. In the presence of a more severe drought, soybean plants may also flower prematurely or have a decreased window of flowering. Furthermore, the soybean may abort more flowers and pods than in an average year during high temperatures and limited rainfall. The greatest yield impact of drought in soybeans has been noted when beans are between the growth stages R4 (full pod- pods are .75-inch at one of the four top nodes) and R6 (full seed-pod containing a green seed fills capacity at 1 of 4 top nodes).

As the forecast continues to predict high temperatures and limited rainfall, there is the temptation to apply a product that will help our crops beat this heat. Foliar fertilizers, plant growth hormones, amino acids, and other biostimulants can temporarily perk up the crop but ultimately a timely rainfall is the best remedy for hot, droughty conditions.  

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